It is a well known aim for designers of portable devices to minimize the power consumption of such devices, e.g. radio telephones, in order to increase the interval between the charging and replacing of the batteries for the portable devices. Additionally, the likelihood of the portable device ceasing operation during a working cycle is reduced if power consumption is minimized. Therefore, portable devices, particularly radio telephones, are designed so that different power-consuming parts can be shut off when not required so that only the necessary parts required to "activate" the portable device are switched on. In a portable communication system, the portable devices are paged by radio transmitted paging messages. The so-called paging messages are transmitted using paging channels of the communication system to the portable devices, e.g. cellular telephone, inside a given cell in different systems. The cellular telephone recognizes, on the basis of these messages, whether or not there is an incoming call for that cellular telephone i.e., whether certain parts of the cellular telephone should be activated. These paging messages are typically transmitted, for example, as signal blocks, signal bursts or as an FSK-modulation bit stream, using a continuous carrier wave. The particular implementation depends on the communication system. Typically, such communication systems have discontinuous reception (DRX), i.e., parts of the cellular telephone are switched off between paging messages and also during paging messages. This is the case when the cellular telephone is in the so-called idle state (stand-by).
In the stand-by state the power consumption of cellular telephones is mainly due to the receiving and handling of paging messages. Generally the power consumption is reduced by the power being switched off between paging message blocks, but power consumption should be minimized also during paging message blocks or bursts.
In known digital cellular telephones, for example, the reception of paging messages is carried out in the following manner, Paging messages are received and decoded in a digital signal processor (DSP) and the decoded bits are subsequently analyzed in the microprocessor (MCU). A parallel or serial bus is generally provided between the microprocessor and the digital signal processor. A drawback in this process is that the whole reception chain is switched on during or after the reception of a paging message block. Additionally, the majority of paging messages do not require any activity from the cellular telephone because they are either so-called empty messages or paging messages for other cellular telephones.
A known improvement on the above method is one where relevant paging messages are partly detected in the signal processor DSP and any blocks which require activity are then forwarded to the microprocessor for full processing. A drawback here is that because the signal processor has to transmit the information to the microprocessor, this information is written into the memory twice, the first time into the memory of the DSP and the second time the same information is written into the memory of the MCU. Therefore, both the microprocessor and the signal processor have to be switched on or they have to use a higher clock frequency for a longer period of time if the microprocessor is one which does not have a halt mode. Another disadvantage is that the MCU and/or its peripheral interface circuits (serial/parallel ports, DMA Direct Memory Access controller, etc.) have to be activated in order to transfer the paging information into the memory of the MCU. The MCU can start processing the data only after the whole message has been transferred to the RAM area for the received messages.